I am trying to upgrade the kernel on my domU's to 3.2.1, currently they are running 2.6.38. The dom0 already runs with 3.2.1, everything is fine with that. But when I boot the domU's with any kernel greater than 2.6.38 (e.g. 3.2.1), they can only access the internal network, but not the internet (or at least reliably).

Pinging an external IP with the new kernel tells me "Destination Host Unreachable" (though sometimes the first pings work), internal IP's work fine.